1. Field of the Invention
The present invention relates to an ink jet recording method and apparatus and an ink jet recording head, in which recording is preformed by ejecting ink out of a recording head and applying it to recording medium.
2. Related Background Art
In recording apparatuses such as printers, copiers, and facsimiles, dots are recorded with recording elements (such as nozzles, heating elements and wires) on recording medium such as paper and plastic thin plate in accordance with image information to thereby record an image composed of dots. Such recording apparatuses are classified based on their recording methods, into an ink jet type, a wire dot type, a thermal type, a laser beam type, and the like. Of these types, the ink jet type (ink jet printer) records an image by ejecting ink (recording liquid) out of an ejection port (nozzle) of a recording head and blowing it onto recording medium.
A number of recording apparatuses are used nowadays with output terminals such as personal computers and image processing apparatuses. These recording apparatuses are required to have functions of high speed recording, high resolution, high image quality, low noises and the like. One example of recording apparatuses which can meet such requirements is an ink jet recording apparatus. Since an ink jet recording apparatus performs recording by ejecting ink out of a recording head, non-contact recording relative to recording medium is possible so that a very stable record image can be formed.
With recent advent of various types of digital cameras, digital camcoders, CD-ROM's and the like, pictorial image data can now be easily processed by applications running on a host computer. Under these circumstances, a performance of outputting pictorial images is required for output apparatuses such as printers. Conventionally, a pictorial image has been recorded by a highly sophisticated silver salt type recording apparatus which uses digital image input or an expensive sublimation type recording apparatus which is limited only to photographic output generated by using sublimation dye.
Conventional such recording apparatuses dedicated to photographic images are very expensive. One reason is a very complicated process of the silver salt type and a large size unsuitable for desk-top use. Another reason is use of sublimation dye by the sublimation type apparatus, which results in a larger cost of the apparatus and its larger running cost as the size of recording medium becomes larger. These conventional recording apparatuses are too expensive for general users. The most significant disadvantage is that design of such apparatuses assumes use of specific recording medium. Therefore, these apparatuses are not suitable for use shared by general persons and professionals. It is very cumbersome and difficult to discriminately use between plain paper sheets and specific recording sheets in order to record graphic originals formed by a word processor and pictorial photographic originals.
An ink jet printer is known as a recording apparatus which reduces such limitations on recording media. In order to solve the above problems associated with such ink jet printers, image processing, coloring agents and recording media have been improved and a photographic image with a considerably improved quality can now be printed.
Several studies have been made in order to improve the tonal level of a color graphic output. For example, those improvements proposed recently in practical use include a record resolution improved more than a normal color recording mode to provide a better drawing capability, a multi-value output using sub-pixels with an improved record resolution, and the like.
Another practical recording method is to uniformly reduce an amount of ejected ink during a high resolution mode by changing an ink ejection amount of a recording head. Recording heads such as those capable of modulating an ink ejection amount at each nozzle have also been proposed.
The above-described conventional recording methods are associated with, however, the following problems.
(1) The method of uniformly reducing ink jet amount records an image at a higher resolution both in the main and subsidiary scan directions. Therefore, the number of main scans increases and the feed amount in the sub-scan direction reduces so that the recording speed lowers greatly. As the resolution of recording data is raised, the data amount increases greatly which results in a large increase of the memory capacity, increased data transfer amount and time required by interface, an increase of load of a printer driver, and the like. For example, if the resolution of record data is increased by two times, the data amount is doubled for both in the main and sub-scan directions so that the total data amount is a square of 2 or four times. Since recording dots are made fine in order to suppress a granular image quality (irregular image quality) at a low density area, a number of fine dots are also recorded at the high density area although in this area the granular image quality does not become conspicuous. Although the total image quality can be improved, an image forming efficiency is not improved correspondingly.
(2) Another recording method is to use a combination of large and small dots. This method can improve an image forming efficiency. This method can be applied easily if one recording nozzle is used for each color. However, if a plurality of nozzles are used for each color, this method becomes difficult as the number of nozzles increases. Ejection of ink droplets from each nozzle is generally performed at several KHz or higher. If the number of nozzles is small, these nozzles can be controlled directly by a CPU. However, as the number of nozzles increases, it becomes necessary to use hardware such as gate array circuits in addition to the operation of CPU in view of a processing speed. In order to modulate the ink ejection amount of large and small dots, either an ejection drive pulse is modulated or an ejection drive element in a nozzle is changed.
If the ejection element is to be changed, it is necessary to provide the recording head with registers for large and small dots. The number of necessary registers is an integer multiple relative to a record resolution so that the circuit scale of the recording head becomes large and the cost of the recording head rises. If the drive pulse is to be modulated, signal lines are required for independently controlling respective nozzles. As opposed to one signal line, several hundreds of signal lines (as many as the number of nozzles) are required. In this case, other elements such as signal line contacts, a flexible cable to the recording head, recording element driver transistors and the like are also required, leading to increased cost.
If a combination of large and small dots is not recorded during one scan of a recording head, the recording head must be scanned several times for a large dot scan and a small dot scan. With this method, a combination of large and small dots can be recorded with simple circuit structure. However, this method necessarily requires a plurality of scans (hereinafter called multi-path scan). For example, even if small dots are recorded at most of addresses during one scan and only one large dot is recorded during this one scan, a total of two recording scans is necessary irrespective of only one large dot. Furthermore, as the number of multi-path scans or records increases, the record time prolongs so that it is necessary to minimize the number of multi-path records. In this connection, consider that a gradation from low density (white) to high density (black) is reproduced with a two-path record. Recording starts first from the smallest dots when color (including grey scale) develops after the low density area. As the image density increases, small dots are sequentially recorded at available lattice points (virtual record dot positions). After small dots are recorded fully, an image is recorded with mixed dots of large and small dots, and as the image density further increases, large dots are additionally recorded to the maximum density.
For the above record control, the recording apparatus is configured so that large and small dots are recorded alternately between respective multi-path scans. Recording under these conditions may result in a wasteful scan if there is no large dot to be recorded because of small dots recorded at all available lattice points. In addition to this problem, the prevention effect of so-called banding which is characteristic to the multi-path divisional recording is lost, because the recording is performed 100% only by small dots during one scan of the two-path scans. The banding is phenomena of variation of ejection amounts of recording nozzles, and variation of paper feed amounts and the like. Still further, since the record ratio between scans is not uniform, several problems occur such as an inability of lowering an error rate during a scan with a higher record ratio because of different record ratios, an inability of lowering consumption power because of a high instantaneous power during a scan with a higher record ratio, and the like.